The D. Melanogaster Gene Scarlet, Its Associated Phenotypic Characteristics

Introduction

For most of the past century, D. melanogaster has been studied extensively not just to increase our knowledge of its anatomy and physiology but also to establish biological principles that can be applied in many other instances (Roberts 2006). D. melanogaster has become a model organism for research because of the significant level of similarity between D. melanogaster and humans in terms of genomic structure and function. This genomic similarity between humans and D. melanogaster has spurred on numerous comparative studies to gain insight into the pathology or processes involved in many human disorders (Tickoo and Russell 2002). This review explores the D. melanogaster gene, scarlet, whose phenotype (bright vermillion compound eye color) was first observed in a wild type stock and isolated for study by Mildred Richards in 1918. His study of the mutant fly revealed that scarlet is located on chromosome 3 (Richards 1918). scarlet has since been studied because it has a homolog in a family of genes present in but not exclusive to mammals: the ATP-Binding Cassette (ABC) transporter gene super-family. scarlet is one of the genes critical to the formation of the ABC transporter super-family, a group of primary active transporters responsible for the movement of various kinds of molecules across the membrane mainly through the hydrolysis of ATP (Dermauw and Leeuwen 2014). scarlet is generally associated with the transport of eye pigment precursors in the eye cells of D. melanogaster flies mainly because the most visible phenotype associated with the gene is the bright vermillion eyes of flies expressing scarlet (Gramates et al. 2017). While this is indeed a function, scarlet is also involved in other biological processes such as amine transmembrane transporter activity, ATPase activity, and transmembrane co-transport of substances (Gramates et al. 2017).

Phenotypic Characteristics of Mutations in the Gene

Flies expressing scarlet display various phenotypes not seen in wild type flies. These phenotypic characteristics include bright vermillion compound eyes that darken with age, colorless ocelli, reduced uptake of kynurenine, and reduced phenoxazinone synthase activity (Gramates et al. 2017). Numerous mutations in scarlet have been observed and grouped into classes based on the phenotypic characteristics they give rise to. The classes are lethal & recessive, viable, viable with, sterile, viable, eye color defective, eye color defective & heat sensitive, visible & heat sensitive, visible & recessive, visible & recessive & heat sensitive, pigment cell, and pigment cell & heat sensitive (Gramates et al. 2017).

As the names of the classes suggest, some mutations give rise to more than one phenotype. For example, the mutant allele st7 is grouped under the “lethal & recessive” because it has been observed that the lethality associated with this allele is seen only in homozygous flies (Lindsley and Zimm 1992). Another class of mutant alleles is “eye color defective & heat sensitive”. Flies homozygous for alleles in this class are observed to have varying eye color depending on the temperature. For example, flies homozygous for the mutant allele, st12, have been observed to exhibit the bright vermillion eyes of scarlet flies at 29oC but display the bright orange eyes of wild-type flies at 18oC (Howells 1979). In some cases, incomplete dominance can be observed between different scarlet mutants. st14 is a mutant allele that gives rise to a red compound eye color in homozygous flies distinguishable from the bright vermillion eyes of scarlet (Belote et al. 1990). Another mutant allele, st1, is a recessive allele that gives rise to a phenotype characterized by a pale yellow Malpighian tubule (Beadle 1937). Incomplete dominance is observed in flies heterozygous for these alleles. As opposed to vermillion or bright red compound eyes, trans-heterozygous st1/st14 flies display light yellow eyes (Tearle et al. 1989).

The phenotypic classes generally group alleles based on which phenotype they give rise to. However, in order for a phenotype associated with a mutant scarlet allele to be displayed, the flies would have to be homozygous since all the alleles are recessive to the wild type (Gramates et al. 2017). Molecular characteristics of the gene and gene product The function of scarlet can be determined by going down to the molecular level and observing which proteins it encodes for and what those proteins do. scarlet is known to have six protein products: ABC_transporter_CS, ABC_transporter-like, ABC_2_trans type transporter, AAA+_ATPase, pigment_permease/Abcg, and P-loop_NTPase (Gramates et al. 2017). The first three, ABC_transporter_CS, ABC_transporter-like, and ABC_2_trans type transporter, are domains of membrane proteins which are a part of the ABC transporter super-family, transporters that mainly use ATP to direct the movement of various kinds of molecules across either the extracellular or intracellular membrane, thereby regulating activities such as protein export and nutrient uptake.

These domains are present in both mammals and insects (Higgins 1992). The fourth, AAA+_ATPase, is responsible for providing the energy used to drive ABC transporter activity. It accomplishes this by hydrolyzing the β-γ phosphate bond of ATP molecules in its central domain (Snider et al. 2008). P-loop_NTPase is another gene product of scarlet that hydrolyzes high energy bonds to drive ABC transporter activity. P-loop_NTPase is the most common domain found in nucleotide binding proteins. It uses the energy released from the hydrolysis of the β-γ phosphate bond of either ATP or GTP molecules to cause conformational changes in other molecules. In this instance, the energy released is used to cause conformational changes in ABC transporter proteins, thereby driving their activity. (Leipe et al. 2003). The sixth and final known product of scarlet is pigment_permease/Abcg. This is a pigment precursor permease: a trans-membrane protein that is essential for the transport of pigment precursors into pigment cells. In the case of scarlet flies, epistatic interactions between the proteins of white and scarlet contribute to eye color. In the ommochrome synthesis pathway, white protein dimerizes with scarlet protein for the transport of the pigment precursor, tryptophan, into eye pigment cells. Tryptophan is then modified in the pigment cells, thereby producing the bright vermillion eye color observed in scarlet flies. Since the sequence of genes are very specific for the proteins they encode for, it is reasonable to assert that any mutation in scarlet can cause serious problems in the function or formation of the gene products described above.

For example, st14 is caused by a 7. 6 kb insertion and st1 is caused by chromosomal deletion (Gramates et al. 2017). The transcript of these alleles will certainly be different from the transcript of scarlet, leading to entirely different gene products, and by extension, different phenotypes.

Conclusions

The products of scarlet are present and highly conserved in both insects and mammals (Higgins 1992; Snider et al. 2008; Leipe et al. 2003; Dreesen et al. 1988), hinting at how vital these gene products are to various biological functions. The reason scarlet and other D. melanogaster genes like brown and white are spurring on a lot of comparative studies is precisely because they have homologs in the mammalian ABC transporter gene super-family (Dean et al. 2001). Since scarlet encodes for biologically important proteins that have homologs present in mammals, certain mutations in scarlet can be associated with a variety of human disorders that stem from loss of function of any one of their gene products. Some of these disorders include in adrenoleukodystrophy, degeneration of the retina, multi-drug resistance, neurological disease, metabolic defects, endocrine defects, cystic fibrosis, and defects in mitochondrial iron homeostasis (Dean et al. 2001; Dermau et al. 2014; Wightman et al. 2012).

However, research into scarlet is not limited to its function in humans. It has also been suggested that scarlet, in conjunction with other genes, may be involved in the Hedgehog Signaling Pathway, a critical signaling pathway responsible for pattern formation in metazoan animals (Collins and Cohen 2005). The study of scarlet also goes beyond simply identifying mutations responsible for certain disorders. By increasing our understanding of how certain mutations in scarlet alter gene products or biochemical pathways, we may be able to formulate more efficient treatments of disorders that arise as a result of those mutations. Other D. melanogaster genes have been studied to this end, so progress is already being made (Tickoo and Russell 2002).

However, we still need to ponder: what other systems, pathways or disorders is scarlet involved in? For example, ABC genes are known to be associated with fertility and morphology in humans (Dean et al. 2001). Could scarlet be involved in these biological processes that govern fertility and morphology? Future research needs to be conducted to answer these questions. Furthermore, additional research needs to be conducted on scarlet so that we may be able to formulate better treatments of already known disorders related to scarlet based on biochemical pathways or biological processes found to be affected by certain mutations.